Summary During flight, the pilot of the MD Helicopters 369D, serial number 1143D, registration C-GHFL, experienced a vibration and noticed that the main-rotor blades were slightly out of track. When this condition deteriorated such that one of the five blades appeared to be about six inches out of track, the pilot landed the helicopter. Ce rapport est galement disponible en franais. Other Factual Information The helicopter was normally positioned on the deck of a floating barge, where it is subjected to weather. The weather on the days before the occurrence, at the operator's Eclipse Camp near Port Hardy, British Columbia, was reported as very windy. A 300-hour inspection was performed two days before the occurrence, and no anomalies were detected. The day before the occurrence flight, the main-rotor blades were removed to prevent damage from blade flapping because of very high winds. The blades were re-installed on the day of the occurrence; however, the winds were still high, so the blades were tied down when the helicopter was parked. After the occurrence, company maintenance personnel inspected the blades and found that one of the five blades - part number 369D21100-523, serial number 009999-K077, the blade colour-coded yellow - had cracked at the upper and lower skins, from the trailing edge to the spar. The subject main-rotor blade had accumulated 1988 hours between 22April1998 and 12December 2000 while installed on another MD Helicopters 369D, C-GTZR. Torque events1 were not recorded accurately; but beginning 25July1999, the operator started tracking torque events as accurately as possible. Tracking torque events requires a standardized system of recording. Some operators have reported as many as 40torque events per hour. After service on C-GTZR, the blade was installed on the occurrence helicopter.On 26March2001, the failed blade was removed after accumulating a total time since new of 2266.3hours. It is estimated that this blade had had 32523torque events. The failed blade was examined visually and using X-rays, photography, scanning electron microscope, and optical microscope. The examination revealed a fatigue failure and crack that started from a nick or rough radius on the edge of a lightening hole on the C-channel structure of the blade, about three inches inboard of the data plate. The fracture propagated to the top and bottom skins of the blade and subsequently across the cord of the blade, causing a separation at the trailing edge. The fractures stopped at or near the 'D' spar structure near the leading edge of the blade. McDonnell Douglas Helicopter Systems (MDHS, now known as MD Helicopters Inc. [MDHI]) issued Service LetterSL369D-111, dated 11January1999, to establish a new approach to tracking certain retirement lives of various helicopter components. Components such as the main-rotor blades were to be assigned retirement lives based on flight spectrums - flight test, fatigue test, and field experience. Some operators had reported exceeding the estimated average amount of high-stress manoeuvres in the flight spectrum, causing MDHS to re-evaluate the method of establishing limited lives, which only considered time in service. It was proposed that torque events be considered in the flight spectrum equation. Subsequently, on 15May2001, MDHI issued Service Bulletin SB369D-201, annotated as mandatory compliance. On 24July2001, MDHI issued Service Bulletin SB369D-201R1. These bulletins contain criteria to assist operators in understanding the level of usage, the impact of that usage on the main-rotor blade life, and the corresponding inspections required to find cracks that might occur. Compliance with service letters and bulletins is not mandatory under Transport Canada rules. MDHI publishes airworthiness limitations that essentially establish life limits for helicopter components. In accordance with the MD Helicopters, Inc. Maintenance Manual, chapter4, revision29, the life limit for the subject blade, part number 369D21100-523, is 4000 hours in service. The blades have a scheduled inspection interval of 100hours. After a blade has accumulated 750flight hours and 13720torque events, operators are required to perform a main-rotor blade torque-event inspection every 35flight hours or 200torque events, whichever occurs first. No finite torque event number is established for the main rotor blades. During the investigation, MDHI studied 28blades for cracks or other damage. An examination of the C-channel was added to the study protocol. Four of the 28blades showed cracking in the C-channel. Three of the four came from one aircraft involved in operations with a very high number of torque events per hour. The fourth was used in a manner that, although approved for the blade's operations, was outside the normal rotorcraft flight manual limits. The C-channel cracks observed in this study did not appear to be from any manufacturing defect. Blades exposed to a high number of torque events per hour were more likely to display cracks. The study showed that the cracked C-channel found in the Helifor blade was rare but not unique. It also showed that if a crack were present, its progression would be discovered very early with the implementation of Service Bulletin SB369D-201R1. Summary During flight, the pilot of the MD Helicopters 369D, serial number 1143D, registration C-GHFL, experienced a vibration and noticed that the main-rotor blades were slightly out of track. When this condition deteriorated such that one of the five blades appeared to be about six inches out of track, the pilot landed the helicopter. Ce rapport est galement disponible en franais. Other Factual Information The helicopter was normally positioned on the deck of a floating barge, where it is subjected to weather. The weather on the days before the occurrence, at the operator's Eclipse Camp near Port Hardy, British Columbia, was reported as very windy. A 300-hour inspection was performed two days before the occurrence, and no anomalies were detected. The day before the occurrence flight, the main-rotor blades were removed to prevent damage from blade flapping because of very high winds. The blades were re-installed on the day of the occurrence; however, the winds were still high, so the blades were tied down when the helicopter was parked. After the occurrence, company maintenance personnel inspected the blades and found that one of the five blades - part number 369D21100-523, serial number 009999-K077, the blade colour-coded yellow - had cracked at the upper and lower skins, from the trailing edge to the spar. The subject main-rotor blade had accumulated 1988 hours between 22April1998 and 12December 2000 while installed on another MD Helicopters 369D, C-GTZR. Torque events1 were not recorded accurately; but beginning 25July1999, the operator started tracking torque events as accurately as possible. Tracking torque events requires a standardized system of recording. Some operators have reported as many as 40torque events per hour. After service on C-GTZR, the blade was installed on the occurrence helicopter.On 26March2001, the failed blade was removed after accumulating a total time since new of 2266.3hours. It is estimated that this blade had had 32523torque events. The failed blade was examined visually and using X-rays, photography, scanning electron microscope, and optical microscope. The examination revealed a fatigue failure and crack that started from a nick or rough radius on the edge of a lightening hole on the C-channel structure of the blade, about three inches inboard of the data plate. The fracture propagated to the top and bottom skins of the blade and subsequently across the cord of the blade, causing a separation at the trailing edge. The fractures stopped at or near the 'D' spar structure near the leading edge of the blade. McDonnell Douglas Helicopter Systems (MDHS, now known as MD Helicopters Inc. [MDHI]) issued Service LetterSL369D-111, dated 11January1999, to establish a new approach to tracking certain retirement lives of various helicopter components. Components such as the main-rotor blades were to be assigned retirement lives based on flight spectrums - flight test, fatigue test, and field experience. Some operators had reported exceeding the estimated average amount of high-stress manoeuvres in the flight spectrum, causing MDHS to re-evaluate the method of establishing limited lives, which only considered time in service. It was proposed that torque events be considered in the flight spectrum equation. Subsequently, on 15May2001, MDHI issued Service Bulletin SB369D-201, annotated as mandatory compliance. On 24July2001, MDHI issued Service Bulletin SB369D-201R1. These bulletins contain criteria to assist operators in understanding the level of usage, the impact of that usage on the main-rotor blade life, and the corresponding inspections required to find cracks that might occur. Compliance with service letters and bulletins is not mandatory under Transport Canada rules. MDHI publishes airworthiness limitations that essentially establish life limits for helicopter components. In accordance with the MD Helicopters, Inc. Maintenance Manual, chapter4, revision29, the life limit for the subject blade, part number 369D21100-523, is 4000 hours in service. The blades have a scheduled inspection interval of 100hours. After a blade has accumulated 750flight hours and 13720torque events, operators are required to perform a main-rotor blade torque-event inspection every 35flight hours or 200torque events, whichever occurs first. No finite torque event number is established for the main rotor blades. During the investigation, MDHI studied 28blades for cracks or other damage. An examination of the C-channel was added to the study protocol. Four of the 28blades showed cracking in the C-channel. Three of the four came from one aircraft involved in operations with a very high number of torque events per hour. The fourth was used in a manner that, although approved for the blade's operations, was outside the normal rotorcraft flight manual limits. The C-channel cracks observed in this study did not appear to be from any manufacturing defect. Blades exposed to a high number of torque events per hour were more likely to display cracks. The study showed that the cracked C-channel found in the Helifor blade was rare but not unique. It also showed that if a crack were present, its progression would be discovered very early with the implementation of Service Bulletin SB369D-201R1.